Animation API summary link

The functional API provided by the @angular/animations module provides a domain-specific language (DSL) for creating and controlling animations in Angular applications. See the API reference for a complete listing and syntax details of the core functions and related data structures.

Animation transition and timing link

An animation transition specifies changes that occur between one state and another. Set the transition to make the change less abrupt. An animation transition specifies the changes that occur between one state and another.

Animation transition() function defined link

The transition() function accepts two arguments:

An expression that defines the direction between two transition states An expression that accepts one or a series of animate() steps

Use the animate() function of a transition to define:

• Length
• Delay
• Easing
• Style function for defining styles while transitions are taking place

Use the animate() function to define the keyframes() function for multi-step animations. These definitions are placed in the second argument of the animate() function.

Animation metadata: duration, delay, and easing link

The animate() function accepts the timings and styles input parameters.

The timings parameter takes either a number or a string defined in three parts.

      
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      animate (duration)
    

or

      
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      animate ('duration delay easing')
    

The first part, duration , is required. The duration can be expressed in milliseconds as a number without quotes, or in seconds with quotes and a time specifier. For example, a duration of a tenth of a second can be expressed as follows:

• As a plain number, in milliseconds: 100

• In a string, as milliseconds: '100ms'

• In a string, as seconds: '0.1s'

The second argument, delay , has the same syntax as duration . For example:

• Wait for 100 ms and then run for 200 ms: '0.2s 100ms'

The third argument, easing , controls how the animation accelerates and decelerates during its runtime. For example, ease-in causes the animation to begin slowly, and to pick up speed as it progresses.

• Wait for 100 ms, run for 200 ms. Use a deceleration curve to start out fast and slowly decelerate to a resting point:

        
          content_copy
        
        0.2s 100ms ease-out
      

• Run for 200 ms, with no delay. Use a standard curve to start slow, speed up in the middle, and then decelerate slowly at the end:

        
          content_copy
        
        0.2s ease-in-out
      

• Start immediately, run for 200 ms. Use an acceleration curve to start slow and end at full velocity:

        
          content_copy
        
        
  The code sample is missing.
      

  0.2s ease-in

This example provides a state transition from open to closed with a 1-second transition between states.

      
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      transition('open => closed', [
  animate('1s')
]),
    

In the preceding code snippet, the => operator indicates unidirectional transitions, and <=> is bidirectional. Within the transition, animate() specifies how long the transition takes. In this case, the state change from open to closed takes 1 second, expressed here as 1s .

This example adds a state transition from the closed state to the open state with a 0.5-second transition animation arc.

      
        content_copy
      
      transition('closed => open', [
  animate('0.5s')
]),
    

Define animations and attach to an HTML template link

Animations are defined in the metadata of the component that controls the HTML element to be animated.

Define the animation link

Put the code that defines your animations under the animations: property within the @Component() decorator.

      
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      @Component({
  selector: 'app-open-close',
  animations: [
    trigger('openClose', [
      // ...
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.8,
        backgroundColor: 'blue'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'open-close.component.html',
  styleUrls: ['open-close.component.css']
})
export class OpenCloseComponent {
  isOpen = true;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}
    

When an animation trigger for a component is defined, attach it to an element in the template. Wrap the trigger name in brackets and precede it with an @ symbol. Bind the trigger to a template expression using standard Angular property binding syntax. The triggerName is the name of the trigger, and expression evaluates to a defined animation state.

      
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      <div [@triggerName]="expression">…</div>;
    

The animation is executed or triggered when the expression value changes to a new state.

The following code snippet binds the trigger to the value of the isOpen property.

      
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      <nav>
  <button type="button" (click)="toggle()">Toggle Open/Close</button>
</nav>

<div [@openClose]="isOpen ? 'open' : 'closed'" class="open-close-container">
  <p>The box is now {{ isOpen ? 'Open' : 'Closed' }}!</p>
</div>
    

In this example, when the isOpen expression evaluates to a defined state of open or closed , it notifies the trigger openClose of a state change. Then it's up to the openClose code to handle the state change and kick off a state change animation.

For elements entering or leaving a page (inserted or removed from the DOM), you can make the animations conditional. For example, use *ngIf with the animation trigger in the HTML template.

Introduction to Angular animations link

Animation provides the illusion of motion: HTML elements change styling over time. Well-designed animations can make your application more fun and straightforward to use, but they aren't just cosmetic. Animations can improve your application and user experience in a number of ways:

• Without animations, web page transitions can seem abrupt and jarring
• Motion greatly enhances the user experience, so animations give users a chance to detect the application's response to their actions
• Good animations intuitively call the user's attention to where it is needed

Typically, animations involve multiple style transformations over time. An HTML element can move, change color, grow or shrink, fade, or slide off the page. These changes can occur simultaneously or sequentially. You can control the timing of each transformation.

Angular's animation system is built on CSS functionality, which means you can animate any property that the browser considers animatable. This includes positions, sizes, transforms, colors, borders, and more. The W3C maintains a list of animatable properties on its CSS Transitions page.

About this guide link

This guide covers the basic Angular animation features to get you started on adding Angular animations to your project.

The features described in this guide —and the more advanced features described in the related Angular animations guides— are demonstrated in an example application available as a live example / download example .

Prerequisites link

The guide assumes that you're familiar with building basic Angular apps, as described in the following sections:

• Tutorial
• Architecture Overview

Getting started link

The main Angular modules for animations are @angular/animations and @angular/platform-browser . When you create a new project using the CLI, these dependencies are automatically added to your project.

To get started with adding Angular animations to your project, import the animation-specific modules along with standard Angular functionality.

Step 1: Enabling the animations module link

Import BrowserAnimationsModule , which introduces the animation capabilities into your Angular root application module.

      
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      import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

@NgModule({
  imports: [
    BrowserModule,
    BrowserAnimationsModule
  ],
  declarations: [ ],
  bootstrap: [ ]
})
export class AppModule { }
    

Step 2: Importing animation functions into component files link

If you plan to use specific animation functions in component files, import those functions from @angular/animations .

      
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      import { Component, HostBinding } from '@angular/core';
import {
  trigger,
  state,
  style,
  animate,
  transition,
  // ...
} from '@angular/animations';
    

Step 3: Adding the animation metadata property link

In the component file, add a metadata property called animations: within the @Component() decorator. You put the trigger that defines an animation within the animations metadata property.

      
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      @Component({
  selector: 'app-root',
  templateUrl: 'app.component.html',
  styleUrls: ['app.component.css'],
  animations: [
    // animation triggers go here
  ]
})
    

Animating a transition link

Let's animate a transition that changes a single HTML element from one state to another. For example, you can specify that a button displays either Open or Closed based on the user's last action. When the button is in the open state, it's visible and yellow. When it's the closed state, it's translucent and blue.

In HTML, these attributes are set using ordinary CSS styles such as color and opacity. In Angular, use the style() function to specify a set of CSS styles for use with animations. Collect a set of styles in an animation state, and give the state a name, such as open or closed .

      
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      ng g component open-close
    

Animation state and styles link

Use Angular's state() function to define different states to call at the end of each transition. This function takes two arguments: A unique name like open or closed and a style() function.

Use the style() function to define a set of styles to associate with a given state name. You must use camelCase for style attributes that contain dashes, such as backgroundColor or wrap them in quotes, such as 'background-color' .

Let's see how Angular's state() function works with the style⁣­(⁠) function to set CSS style attributes. In this code snippet, multiple style attributes are set at the same time for the state. In the open state, the button has a height of 200 pixels, an opacity of 1, and a yellow background color.

      
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      // ...
state('open', style({
  height: '200px',
  opacity: 1,
  backgroundColor: 'yellow'
})),
    

In the following closed state, the button has a height of 100 pixels, an opacity of 0.8, and a background color of blue.

      
        content_copy
      
      state('closed', style({
  height: '100px',
  opacity: 0.8,
  backgroundColor: 'blue'
})),
    

Transitions and timing link

In Angular, you can set multiple styles without any animation. However, without further refinement, the button instantly transforms with no fade, no shrinkage, or other visible indicator that a change is occurring.

To make the change less abrupt, you need to define an animation transition to specify the changes that occur between one state and another over a period of time. The transition() function accepts two arguments: The first argument accepts an expression that defines the direction between two transition states, and the second argument accepts one or a series of animate() steps.

Use the animate() function to define the length, delay, and easing of a transition, and to designate the style function for defining styles while transitions are taking place. Use the animate() function to define the keyframes() function for multi-step animations. These definitions are placed in the second argument of the animate() function.

Animation metadata: duration, delay, and easing link

The animate() function (second argument of the transition function) accepts the timings and styles input parameters.

The timings parameter takes either a number or a string defined in three parts.

      
        content_copy
      
      animate (duration)
    

or

      
        content_copy
      
      animate ('duration delay easing')
    

The first part, duration , is required. The duration can be expressed in milliseconds as a number without quotes, or in seconds with quotes and a time specifier. For example, a duration of a tenth of a second can be expressed as follows:

• As a plain number, in milliseconds: 100

• In a string, as milliseconds: '100ms'

• In a string, as seconds: '0.1s'

The second argument, delay , has the same syntax as duration . For example:

• Wait for 100ms and then run for 200ms: '0.2s 100ms'

The third argument, easing , controls how the animation accelerates and decelerates during its runtime. For example, ease-in causes the animation to begin slowly, and to pick up speed as it progresses.

• Wait for 100ms, run for 200ms. Use a deceleration curve to start out fast and slowly decelerate to a resting point: '0.2s 100ms ease-out'

• Run for 200ms, with no delay. Use a standard curve to start slow, accelerate in the middle, and then decelerate slowly at the end: '0.2s ease-in-out'

• Start immediately, run for 200ms. Use an acceleration curve to start slow and end at full velocity: '0.2s ease-in'

This example provides a state transition from open to closed with a 1-second transition between states.

      
        content_copy
      
      transition('open => closed', [
  animate('1s')
]),
    

In the preceding code snippet, the => operator indicates unidirectional transitions, and <=> is bidirectional. Within the transition, animate() specifies how long the transition takes. In this case, the state change from open to closed takes 1 second, expressed here as 1s .

This example adds a state transition from the closed state to the open state with a 0.5-second transition animation arc.

      
        content_copy
      
      transition('closed => open', [
  animate('0.5s')
]),
    

Triggering the animation link

An animation requires a trigger , so that it knows when to start. The trigger() function collects the states and transitions, and gives the animation a name, so that you can attach it to the triggering element in the HTML template.

The trigger() function describes the property name to watch for changes. When a change occurs, the trigger initiates the actions included in its definition. These actions can be transitions or other functions, as we'll see later on.

In this example, we'll name the trigger openClose , and attach it to the button element. The trigger describes the open and closed states, and the timings for the two transitions.

Defining animations and attaching them to the HTML template link

Animations are defined in the metadata of the component that controls the HTML element to be animated. Put the code that defines your animations under the animations: property within the @Component() decorator.

      
        content_copy
      
      @Component({
  selector: 'app-open-close',
  animations: [
    trigger('openClose', [
      // ...
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.8,
        backgroundColor: 'blue'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'open-close.component.html',
  styleUrls: ['open-close.component.css']
})
export class OpenCloseComponent {
  isOpen = true;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}
    

When you've defined an animation trigger for a component, attach it to an element in that component's template by wrapping the trigger name in brackets and preceding it with an @ symbol. Then, you can bind the trigger to a template expression using standard Angular property binding syntax as shown below, where triggerName is the name of the trigger, and expression evaluates to a defined animation state.

      
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      <div [@triggerName]="expression">…</div>;
    

The animation is executed or triggered when the expression value changes to a new state.

The following code snippet binds the trigger to the value of the isOpen property.

      
        content_copy
      
      <nav>
  <button type="button" (click)="toggle()">Toggle Open/Close</button>
</nav>

<div [@openClose]="isOpen ? 'open' : 'closed'" class="open-close-container">
  <p>The box is now {{ isOpen ? 'Open' : 'Closed' }}!</p>
</div>
    

In this example, when the isOpen expression evaluates to a defined state of open or closed , it notifies the trigger openClose of a state change. Then it's up to the openClose code to handle the state change and kick off a state change animation.

For elements entering or leaving a page (inserted or removed from the DOM), you can make the animations conditional. For example, use *ngIf with the animation trigger in the HTML template.

Code review link

Here are the code files discussed in the transition example.

      
        content_copy
      
      @Component({
  selector: 'app-open-close',
  animations: [
    trigger('openClose', [
      // ...
      state('open', style({
        height: '200px',
        opacity: 1,
        backgroundColor: 'yellow'
      })),
      state('closed', style({
        height: '100px',
        opacity: 0.8,
        backgroundColor: 'blue'
      })),
      transition('open => closed', [
        animate('1s')
      ]),
      transition('closed => open', [
        animate('0.5s')
      ]),
    ]),
  ],
  templateUrl: 'open-close.component.html',
  styleUrls: ['open-close.component.css']
})
export class OpenCloseComponent {
  isOpen = true;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}
    

Summary link

You learned to add animation to a transition between two states, using style() and state() along with animate() for the timing.

Learn about more advanced features in Angular animations under the Animation section, beginning with advanced techniques in transition and triggers.

Animations API summary link

The functional API provided by the @angular/animations module provides a domain-specific language (DSL) for creating and controlling animations in Angular applications. See the API reference for a complete listing and syntax details of the core functions and related data structures.

More on Angular animations link

You might also be interested in the following:

• Transition and triggers
• Complex animation sequences
• Reusable animations
• Route transition animations

Ahead-of-time (AOT) compilation link

An Angular application consists mainly of components and their HTML templates. Because the components and templates provided by Angular cannot be understood by the browser directly, Angular applications require a compilation process before they can run in a browser.

The Angular ahead-of-time (AOT) compiler converts your Angular HTML and TypeScript code into efficient JavaScript code during the build phase before the browser downloads and runs that code. Compiling your application during the build process provides a faster rendering in the browser.

This guide explains how to specify metadata and apply available compiler options to compile your applications efficiently using the AOT compiler.

Here are some reasons you might want to use AOT.

Choosing a compiler link

Angular offers two ways to compile your application:

When you run the ng build (build only) or ng serve (build and serve locally) CLI commands, the type of compilation (JIT or AOT) depends on the value of the aot property in your build configuration specified in angular.json . By default, aot is set to true for new CLI applications.

See the CLI command reference and Building and serving Angular apps for more information.

How AOT works link

The Angular AOT compiler extracts metadata to interpret the parts of the application that Angular is supposed to manage. You can specify the metadata explicitly in decorators such as @Component() and @Input() , or implicitly in the constructor declarations of the decorated classes. The metadata tells Angular how to construct instances of your application classes and interact with them at runtime.

In the following example, the @Component() metadata object and the class constructor tell Angular how to create and display an instance of TypicalComponent .

      
        content_copy
      
      @Component({
  selector: 'app-typical',
  template: '<div>A typical component for {{data.name}}</div>'
})
export class TypicalComponent {
  @Input() data: TypicalData;
  constructor(private someService: SomeService) { … }
}
    

The Angular compiler extracts the metadata once and generates a factory for TypicalComponent . When it needs to create a TypicalComponent instance, Angular calls the factory, which produces a new visual element, bound to a new instance of the component class with its injected dependency.

Compilation phases link

There are three phases of AOT compilation.

Metadata restrictions link

You write metadata in a subset of TypeScript that must conform to the following general constraints:

• Limit expression syntax to the supported subset of JavaScript
• Only reference exported symbols after code folding
• Only call functions supported by the compiler
• Decorated and data-bound class members must be public

For additional guidelines and instructions on preparing an application for AOT compilation, see Angular: Writing AOT-friendly applications.

Configuring AOT compilation link

You can provide options in the TypeScript configuration file that controls the compilation process. See Angular compiler options for a complete list of available options.

Phase 1: Code analysis link

The TypeScript compiler does some of the analytic work of the first phase. It emits the .d.ts type definition files with type information that the AOT compiler needs to generate application code. At the same time, the AOT collector analyzes the metadata recorded in the Angular decorators and outputs metadata information in .metadata.json files, one per .d.ts file.

You can think of .metadata.json as a diagram of the overall structure of a decorator's metadata, represented as an abstract syntax tree (AST).

Expression syntax limitations link

The AOT collector only understands a subset of JavaScript. Define metadata objects with the following limited syntax:

If an expression uses unsupported syntax, the collector writes an error node to the .metadata.json file. The compiler later reports the error if it needs that piece of metadata to generate the application code.

      
        content_copy
      
      "angularCompilerOptions": {
  …
  "strictMetadataEmit" : true
}
    

No arrow functions link

The AOT compiler does not support function expressions and arrow functions, also called lambda functions.

Consider the following component decorator:

      
        content_copy
      
      @Component({
  …
  providers: [{provide: server, useFactory: () => new Server()}]
})
    

The AOT collector does not support the arrow function, () => new Server() , in a metadata expression. It generates an error node in place of the function. When the compiler later interprets this node, it reports an error that invites you to turn the arrow function into an exported function .

You can fix the error by converting to this:

      
        content_copy
      
      export function serverFactory() {
  return new Server();
}

@Component({
  …
  providers: [{provide: server, useFactory: serverFactory}]
})
    

In version 5 and later, the compiler automatically performs this rewriting while emitting the .js file.

Code folding link

The compiler can only resolve references to exported symbols. The collector, however, can evaluate an expression during collection and record the result in the .metadata.json , rather than the original expression. This allows you to make limited use of non-exported symbols within expressions.

For example, the collector can evaluate the expression 1 + 2 + 3 + 4 and replace it with the result, 10 . This process is called folding . An expression that can be reduced in this manner is foldable .

The collector can evaluate references to module-local const declarations and initialized var and let declarations, effectively removing them from the .metadata.json file.

Consider the following component definition:

      
        content_copy
      
      const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template
})
export class HeroComponent {
  @Input() hero: Hero;
}
    

The compiler could not refer to the template constant because it isn't exported. The collector, however, can fold the template constant into the metadata definition by in-lining its contents. The effect is the same as if you had written:

      
        content_copy
      
      @Component({
  selector: 'app-hero',
  template: '<div>{{hero.name}}</div>'
})
export class HeroComponent {
  @Input() hero: Hero;
}
    

There is no longer a reference to template and, therefore, nothing to trouble the compiler when it later interprets the collector's output in .metadata.json .

You can take this example a step further by including the template constant in another expression:

      
        content_copy
      
      const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template + '<div>{{hero.title}}</div>'
})
export class HeroComponent {
  @Input() hero: Hero;
}
    

The collector reduces this expression to its equivalent folded string:

      
        content_copy
      
      '<div>{{hero.name}}</div><div>{{hero.title}}</div>'
    

Foldable syntax link

The following table describes which expressions the collector can and cannot fold:

If an expression is not foldable, the collector writes it to .metadata.json as an AST for the compiler to resolve.

Phase 2: code generation link

The collector makes no attempt to understand the metadata that it collects and outputs to .metadata.json . It represents the metadata as best it can and records errors when it detects a metadata syntax violation. It's the compiler's job to interpret the .metadata.json in the code generation phase.

The compiler understands all syntax forms that the collector supports, but it may reject syntactically correct metadata if the semantics violate compiler rules.

Public symbols link

The compiler can only reference exported symbols .

• Decorated component class members must be public. You cannot make an @Input() property private or protected.

• Data bound properties must also be public

Supported classes and functions link

The collector can represent a function call or object creation with new as long as the syntax is valid. The compiler, however, can later refuse to generate a call to a particular function or creation of a particular object.

The compiler can only create instances of certain classes, supports only core decorators, and only supports calls to macros (functions or static methods) that return expressions.

Functions and static method calls link

The collector accepts any function or static method that contains a single return statement. The compiler, however, only supports macros in the form of functions or static methods that return an expression .

For example, consider the following function:

      
        content_copy
      
      export function wrapInArray<T>(value: T): T[] {
  return [value];
}
    

You can call the wrapInArray in a metadata definition because it returns the value of an expression that conforms to the compiler's restrictive JavaScript subset.

You might use wrapInArray() like this:

      
        content_copy
      
      @NgModule({
  declarations: wrapInArray(TypicalComponent)
})
export class TypicalModule {}
    

The compiler treats this usage as if you had written:

      
        content_copy
      
      @NgModule({
  declarations: [TypicalComponent]
})
export class TypicalModule {}
    

The Angular RouterModule exports two macro static methods, forRoot and forChild , to help declare root and child routes. Review the source code for these methods to see how macros can simplify configuration of complex NgModules.

Metadata rewriting link

The compiler treats object literals containing the fields useClass , useValue , useFactory , and data specially, converting the expression initializing one of these fields into an exported variable that replaces the expression. This process of rewriting these expressions removes all the restrictions on what can be in them because the compiler doesn't need to know the expression's value —it just needs to be able to generate a reference to the value.

You might write something like:

      
        content_copy
      
      class TypicalServer {

}

@NgModule({
  providers: [{provide: SERVER, useFactory: () => TypicalServer}]
})
export class TypicalModule {}
    

Without rewriting, this would be invalid because lambdas are not supported and TypicalServer is not exported. To allow this, the compiler automatically rewrites this to something like:

      
        content_copy
      
      class TypicalServer {

}

export const θ0 = () => new TypicalServer();

@NgModule({
  providers: [{provide: SERVER, useFactory: θ0}]
})
export class TypicalModule {}
    

This allows the compiler to generate a reference to θ0 in the factory without having to know what the value of θ0 contains.

The compiler does the rewriting during the emit of the .js file. It does not, however, rewrite the .d.ts file, so TypeScript doesn't recognize it as being an export. And it does not interfere with the ES module's exported API.

Phase 3: Template type checking link

One of the Angular compiler's most helpful features is the ability to type-check expressions within templates, and catch any errors before they cause crashes at runtime. In the template type-checking phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates.

Enable this phase explicitly by adding the compiler option "fullTemplateTypeCheck" in the "angularCompilerOptions" of the project's TypeScript configuration file (see Angular Compiler Options).

Template validation produces error messages when a type error is detected in a template binding expression, similar to how type errors are reported by the TypeScript compiler against code in a .ts file.

For example, consider the following component:

      
        content_copy
      
      @Component({
  selector: 'my-component',
  template: '{{person.addresss.street}}'
})
class MyComponent {
  person?: Person;
}
    

This produces the following error:

      
      my.component.ts.MyComponent.html(1,1): : Property 'addresss' does not exist on type 'Person'. Did you mean 'address'?
    

The file name reported in the error message, my.component.ts.MyComponent.html , is a synthetic file generated by the template compiler that holds contents of the MyComponent class template. The compiler never writes this file to disk. The line and column numbers are relative to the template string in the @Component annotation of the class, MyComponent in this case. If a component uses templateUrl instead of template , the errors are reported in the HTML file referenced by the templateUrl instead of a synthetic file.

The error location is the beginning of the text node that contains the interpolation expression with the error. If the error is in an attribute binding such as [value]="person.address.street" , the error location is the location of the attribute that contains the error.

The validation uses the TypeScript type checker and the options supplied to the TypeScript compiler to control how detailed the type validation is. For example, if the strictTypeChecks is specified, the error

      
      my.component.ts.MyComponent.html(1,1): : Object is possibly 'undefined'
    

is reported as well as the above error message.

Type narrowing link

The expression used in an ngIf directive is used to narrow type unions in the Angular template compiler, the same way the if expression does in TypeScript. For example, to avoid Object is possibly 'undefined' error in the template above, modify it to only emit the interpolation if the value of person is initialized as shown below:

      
        content_copy
      
      @Component({
  selector: 'my-component',
  template: ' {{person.address.street}} '
})
class MyComponent {
  person?: Person;
}
    

Using *ngIf allows the TypeScript compiler to infer that the person used in the binding expression will never be undefined .

For more information about input type narrowing, see Improving template type checking for custom directives.

Non-null type assertion operator link

Use the non-null type assertion operator to suppress the Object is possibly 'undefined' error when it is inconvenient to use *ngIf or when some constraint in the component ensures that the expression is always non-null when the binding expression is interpolated.

In the following example, the person and address properties are always set together, implying that address is always non-null if person is non-null. There is no convenient way to describe this constraint to TypeScript and the template compiler, but the error is suppressed in the example by using address!.street .

      
        content_copy
      
      @Component({
  selector: 'my-component',
  template: '<span *ngIf="person"> {{person.name}} lives on {{address!.street}} </span>'
})
class MyComponent {
  person?: Person;
  address?: Address;

  setData(person: Person, address: Address) {
    this.person = person;
    this.address = address;
  }
}
    

The non-null assertion operator should be used sparingly as refactoring of the component might break this constraint.

In this example it is recommended to include the checking of address in the *ngIf as shown below:

      
        content_copy
      
      @Component({
  selector: 'my-component',
  template: '<span *ngIf="person && address"> {{person.name}} lives on {{address.street}} </span>'
})
class MyComponent {
  person?: Person;
  address?: Address;

  setData(person: Person, address: Address) {
    this.person = person;
    this.address = address;
  }
}